In a Calvin cycle 3CO₂ is combines with 3 RUBP to form 6 molecules of glyceraldehyde-3-phosphate (G3P). One molecule of G3P has three fixed carbon atoms, Thus to make one molecule of six carbon glucose molecules will take two molecules of G3P. Therefore six turns of the Calvin cycle required to make one glucose.
So, as per question, 6 molecules of G3P will required 18 carbons.
